1. Field of the Invention
The present invention relates to a method and equipment useful in the manufacture of a semiconductor device on a sapphire substrate, more particularly to a method and equipment for use in a fabrication process that requires heating of the sapphire substrate.
2. Description of the Related Art
A semiconductor device fabricated by forming transistors in a thin semiconductor film such as a silicon (Si) film grown epitaxially on a crystalline sapphire (AL2O3) substrate exhibits far better transistor characteristics than a semiconductor device fabricated on a silicon wafer. One effect of the sapphire substrate is reduced parasitic capacitance.
Another advantage of fabrication on a sapphire substrate is that substantially the same fabrication process can be used as for a silicon wafer, so the same fabrication equipment can be used. A silicon-on-sapphire fabrication line can therefore be set up at a low cost.
When the silicon wafer fabrication process is applied to a sapphire wafer, however, problems arise because of the lower thermal conductivity of the sapphire substrate, and because of its transparency, which causes it to absorb infrared radiation and other forms of radiated heat at a lower rate.
A known method of coping with the transparency problem is to form a thin film of light-absorbing material or electrically conductive material on the underside of the sapphire substrate. Fabrication processes that require preheating of the substrate can then be preceded by a step of radiative or resistive heating of the thin film, from which heat is conducted into the substrate. This method is described in, for example, Japanese Patent Application Publication No. 10-70313 (paragraphs 0019-0032 and FIGS. 3 and 4).
This conventional method, however, suffers from a further problem. If the fabrication process that requires preheating of the substrate is carried out in a comparatively low ambient atmospheric temperature, as in the case in atmospheric-pressure chemical vapor deposition (CVD), for example, then when the sapphire substrate is heated by thermal conduction from the thin film on its underside, due to the low thermal conductivity of sapphire, a temperature difference develops between the upper and lower surfaces of the substrate, causing it to warp in a way that makes it higher at the rim than in the center. As a result, the underside of the substrate becomes convex and therefore difficult to hold by vacuum suction, which is the technique generally used in wafer-handling apparatus.
FIGS. 1 to 6 illustrate an experiment performed by the inventor in investigating this problem. A thin round disc of sapphire was used as a sapphire substrate 1. A hot plate 51 with an internal heater 52 was used to heat the sapphire substrate 1. The experimental apparatus also included a room temperature plate 53, which was kept at the ambient atmospheric temperature, that is, at room temperature. The hot plate 51 was heated by the heater 52 to a temperature of 350° C. while the ambient atmospheric temperature was left unchanged. The sapphire substrate 1 was then placed on the hot plate 51 and its warping behavior was observed. In the following description, the substrate surface on which an epitaxial layer has been or will be created for formation of integrated circuit elements is referred to as its first major surface, the opposite surface being referred to as the second major surface.
In the experiment, the second major surface of the sapphire substrate (1) was placed on the upper surface of the hot plate 51 (FIG. 1). Immediately, the flat sapphire substrate 1 began to warp, becoming saucer-shaped with its second major surface convex. After an elapse of approximately ten seconds, the second major surface became so convex that only its central portion touched the upper surface of the hot plate 51 (FIG. 2). The explanation for this is warping is that, due to the low thermal conductivity of the sapphire substrate 1, a temperature gradient developed in the thickness direction of the sapphire substrate 1, so that the heated second major surface was at a higher temperature than the first major surface, which was exposed to the ambient atmospheric temperature. Accordingly, a difference in thermal expansion developed between the first and second major surfaces, and the second major surface expanded more than the first major surface, pulling and lifting the rim of the sapphire substrate 1 upward.
The sapphire substrate 1 was left in this condition for more than five minutes, but the warping remained unchanged (FIG. 3). The explanation is that the central portion of the second major surface of the sapphire substrate 1, which contacted the hot plate 51, remained thermally expanded by the heat, preventing the sapphire substrate 1 from returning to its original flat shape even though its rim, which had moved away from the hot plate 51, was cooling.
The warped sapphire substrate 1 was then removed from the hot plate 51 and placed immediately with its convex second major surface on the upper surface of the room temperature plate 53 (FIG. 4). This caused the direction of warp to reverse, so that the first major surface became convex, the center of the sapphire substrate 1 now being higher than the rim (FIG. 5). The explanation for this behavior is that the central portion of the second major surface of the sapphire substrate 1 was rapidly cooled by contact with the room temperature plate 53 and its temperature became lower than the temperature of the rim. As a result, the central portion of the second major surface contracted, pulling the rim inward and downward, thereby producing a tensile force that lifted the central portion of the sapphire substrate 1.
When left in this condition on the room temperature plate 53, in several minutes the sapphire substrate 1 returned to its original flat shape as shown in FIG. 6, indicating that the entire area of the sapphire substrate 1 had cooled to room temperature.
Next, the hot plate was heated to 380° C. and positioned about one millimeter (1 mm) away from the second major surface of a sapphire substrate to investigate the effect of non-contact heating in an atmospheric environment.
FIG. 7 is a graph illustrating temperature changes in the sapphire substrate as it underwent non-contact heating. The data in FIG. 7 were obtained by attaching thermocouples to the rim of the sapphire substrate and the center of its first major surface and measuring the temperature changes over time. The solid line indicates the temperature at the center, the dotted line indicates the temperature at the rim, and the dot-dash line indicates, for comparison, the temperature at the center of a silicon substrate heated in the same manner.
As can be seen from FIG. 7, the temperature of the sapphire substrate initially increased more slowly than the temperature of the silicon substrate. The temperature at the rim of the sapphire substrate stabilized at about 320° C. Above 200° C., however, the temperature at the center of the sapphire substrate started to increase more rapidly than the temperature at the rim, and around 330° C., the increase became very steep, quickly reaching 380° C., which was the temperature setting of the hot plate. In contrast, the temperature at the center of the silicon substrate did not show the steep temperature change seen at the center of the sapphire substrate.
These heating curves can be explained as follows. Even in non-contact heating, when a sapphire substrate is heated from one side, heat is transferred by air convection from the hot plate to the adjacent sapphire surface, thereby causing a temperature gradient in the thickness direction of the sapphire substrate, the heated surface becoming hotter than the opposite surface, which is exposed to air at room temperature. This temperature difference causes the sapphire substrate to warp so that its second major surface becomes convex as shown in FIG. 2, bringing the central portion of the substrate closer to the hot plate. The temperature in the center of the substrate then increases rapidly, bringing the central portion into contact the hot plate.
In short, when heated from one side in a comparatively low temperature ambient atmospheric, a sapphire substrate will warp, regardless of whether the heating is by contact or not. Similar warping occurs in the method described in Japanese Patent Application Publication No. 10-70313.
The amount of warping caused by a temperature difference between the first and second major surfaces of a sapphire substrate is estimated below on the basis of the thermal expansion curve shown in FIG. 8. The horizontal axis in FIG. 8 represents the temperature of the sapphire substrate and the vertical axis represents the thermal expansion ratio in percent, determined by measuring total length extension as compared with length at a temperature of absolute zero. Thermal expansion in the c-axis direction of the sapphire crystal is shown by the solid line with white data points and thermal expansion in a direction perpendicular to the c-axis is shown by the dotted line with black data points.
The coefficient of thermal expansion, which can be determined as the slope of the curves in FIG. 8, is approximately eight parts per million per degree Celsius (8 ppm/° C.). Accordingly, when a temperature difference of about 60° C. occurs between the first and second major surfaces of a sapphire substrate, the extension difference with respect to the diameter of the sapphire substrate becomes about 480 ppm. For a sapphire substrate six inches in diameter and six hundred twenty-five micrometers (625 μm) thick, this extension difference produces a warp of about 2 mm. More precisely, for a sapphire substrate with these dimensions, a 2-mm warp is equivalent to an extension difference between the first and second major surfaces of about 450 ppm.
In addition, the following problems have been observed, indicating that the extent of warping of a sapphire substrate during preheating is not a simple phenomenon. Moreover, these observations are only two examples.
(1) Warping of up to several tens of micrometers occurs during fabrication of a sapphire substrate. A sapphire substrate that is warped so that its central portion is convex by several tens of micrometers toward the hot plate has a particular tendency to warp further during preheating.
(2) When various films are formed on a sapphire substrate in order to fabricate semiconductor devices on the substrate, the films change the radiant heat absorption coefficient of the substrate, which creates temperature gradients in directions parallel to the substrate surface, causing local warping during preheating.